Converters are nowadays being increasingly used to feed electrical power into a network from local feeding power stations. These converters are particularly suitable for use with variable rotation-speed generators, such as those provided for modern wind energy installations. In this case, conversion to the frequency of the fixed-frequency supply network (normally 50 Hz) is required in order to feed in the wild-frequency electrical power produced by the generator. In the event of disturbances in the network, the phase angle of the voltage can change suddenly. It has been found that sudden phase changes can have similar negative effects on converters to those of voltage dips. In the case of conventional converters, overcurrents can therefore occur in the event of a sudden phase change, thus activating protective devices for the converter, such as its crowbar circuit. The activation of the protective devices changes the braking torque of the converter and of the generator connected to it, thus leading to undesirable reactions on the mechanical part. Torque oscillations can occur at the generator and on its drive train, which can lead to increased wear or even to failure.
Attempts have been made to use special additional circuits to prevent activation of the converter protective devices. For example, a chopper can be provided in an intermediate circuit of the converter which limits overvoltages or overcurrents occurring as a result of sudden phase changes, in the intermediate circuit itself. However, the additional circuits increase the production complexity and therefore increase the price of the converter. Furthermore, their power-handling capability is restricted, as a result of which they are frequently inadequate for high power levels. In addition, if the protective device is switched on frequently, reactions on the mechanical components of the wind energy installation can reduce their life.